Fiber-reinforced composites manufactured by reinforcing plastic materials (called matrix resins) with various synthetic fibers are used widely in automotive application, aviation and space application, sporting and leisure goods application, and general industrial use. Fibers employed for the composites include inorganic fibers, such as carbon fiber, glass fiber and ceramic fiber; and organic fibers, such as aramid fiber, polyamide fiber and polyethylene fiber. These synthetic fibers are usually produced into filament, and later processed into various forms of reinforcement textiles, including a sheeted intermediate material called unidirectional prepreg which is manufactured by applying hot melt resin to fabric and winding it onto a drum, a textile material manufactured by filament winding, woven fabric or chopped fiber.
Reinforcement fibers are often used in a form of chopped fiber cut into 1 to 15 mm long for manufacturing fiber-reinforced composites of thermoplastic resins, such as polyolefin resins, nylon resins, polycarbonate resins, polyacetal resins, ABS resins, polyphenylene sulfide resins and polyetherimide resins, which are included in the matrix resins mentioned above and attract attention because of their good moldability and advantages in recycling. The chopped fiber should have sufficient cohesion when it is knead with a thermoplastic resin to be manufactured into pellets, and chopped fiber having insufficient cohesion cannot be fed constantly to pellet manufacturing. In addition, reinforcement fiber strands having insufficient fiber cohesion sometimes break to deteriorate the properties of resultant fiber-reinforced composites. For preventing such troubles, numbers of techniques for coating reinforcement fibers with sizing agents containing various thermoplastic resins as a base component have been proposed in order to impart optimum cohesion to the reinforcement fibers (see JPA 58-126375, JP A 60-88062, JP A 2003-165849, JP A 2005-42220 and JP A 2009-1954) and widely employed in industrial fields.
On the other hand, reinforcement fibers are increasingly processed into a form called filament pellet or into a unidirectional sheet, tape or fabric to be impregnated with thermoplastic resins and molded in subsequent processes similarly to composite materials of thermosetting resins, in order to effectively achieve desirable properties of reinforcement fibers including tensile strength. In such cases, hot-melt thermoplastic resins should rapidly penetrate into fiber strands, specifically, fill space between single fibers in molding fiber-reinforced composites in order to shorten the molding time and improve the physical properties of resultant composites.
However, thermoplastic matrix resins including inherently nonpolar polyolefin resins are more viscous than thermosetting resins in molten form, and sometimes fail to sufficiently impregnate fiber strands applied with sizing agents disclosed in prior arts due to insufficient wettability on the fiber strands. Thus the resultant composites sometimes have mechanical properties insufficient for meeting demands for composites.
Under such situation, a sizing agent which improves the affinity between sized fiber and matrix resin to firmly bond the fiber and resin has been demanded in the field of fiber-reinforced composites of thermoplastic matrix resins including polyolefin resins.